We are studying exchanges of carbon monoxide between blood and cardiac or skeletal muscle to provide an estimate of mean myoglobin PO2. We have recently shown that it is possible to use this technique to make estimates of mean myoglobin PO2 in skeletal muscle in intact man. We plan to study effects of increased blood HbCO on changes in mean myoglobin PO2 during exercise at VO2Max in man. We are also studying oxygen autoregulation of intracellular PO2 in skeletal muscle.